If it is not metallic, and it really is that heavy, it is a bit problematic. Perhaps it is made of barytes, though your describing it as 'very heavy' suggests that may not be abnormally heavy enough. Cutting it in half would be useful, but I suspect you don't want to do that. If you did, you might find it mass mainly a metallic ore, iron-based if it affects a compass.

The chalk on the east coast, as elsewhere, contains marcasite nodules in places - which would be heavy enough. But they are usually more rusty and with a different surface appearance.

I'm fairly certain its a round flint, i find lots of these while fossil hunting on the chalk beaches.

I believe these ball shaped flints are thought to have formed around sponge debris trapped in the chalk.

If it was cut open you should find a hollow center with a white powdery substance inside called flint meal, it happens to be full of micro fossils, but please don't try breaking it open or cracking it, as its dangerous to hit flint with a hammer or anything hard, it shatters in all directions & these fragments are extremely sharp.

Shelbyshep - I agree with Keith. It looks like flint on the outside (with lots of impact damage). We really need you to give us better guidance on its heaviness. Please could you find a comparably sized piece of say concrete and tell us if your specimen is a little or a lot heavier than it.

The weight on its own doesn't tell me too much. But since you also wrote that it is a bit bigger than a tennis ball, that does allow me to estimate its bulk density (if it is composed of more than one substance, the density of the heavy component will be higher than the bulk density). The density will give us a clue about its metal or metal-ore content.

A tennis ball is 6.7cm in diameter (3.35cm radius).

Its volume is (pi*4*((3.35)^3))/3 = 157.5cm³

If your specimen is the same size, it density is 634/157.5 = 4.0gm/cm³

If, as you say, your specimen is a bit larger, say 7cm diameter, the density comes down to 3.5gm/cm³.

And if it is 8cm diameter, the density comes down further, to 2.4gm/cm³.

The range of densities for most common rocks is 2.3-3.0gm/cm³.

So the actual size of your specimen is quite significant. If it has a diameter of 8cm, it is a relatively 'lightweight' rock. But if it is 7cm in diameter, it is too heavy for plain rock; it must have a heavy mineral component. But it would not be dense enough to be, for instance, pure magnetite (5.2gm/cm³).

I'm surprised you can feel your magnet being repelled. But this is good news.

Some heavy rock specimens turn out to be largely metal or metal ore, which may influence a magnetic field, but which is not magnetic itself. But there are just two minerals that are naturally magnetic - lodestone (a form of magnetite) and pyrrhotite. The latter is only weakly magnetic. I strongly suspect the majority of your specimen is lodestone. You should find that metal objects are attracted to it, as you can see in the photos here - http://en.wikipedia.org/wiki/Lodestone

Now, you say your specimen came from the east coast. I wonder if it came from the wreck of a ship carrying iron ore. That seems to be the source of some magnetite pebbles at Chesil Beach - http://www.southampton.ac.uk/~imw/cheslode.htm